This invention relates to an improvement in a process for preparing a packaging film and, more particularly, to an improvement in a process for preparing a heat-sealable, moisture-proof film having a double copolymer coating.
It is well known in the art to apply vinylidene chloride copolymer coatings to film substrates such as regenerated cellulose and plastic materials to obtain more functional products. For example, heat-sealability, barrier properties and optical properties can be substantially improved by adding coatings to the film substrate. Most of the available vinylidene chloride copolymers, however, offer a compromise between optimum barrier properties and low heat seal temperatures. Those polymers that are good barriers to moisture and oxygen are highly crystalline and require relatively high temperatures, i.e., above about 130.degree.C., and long dwell times to heat seal, whereas those copolymers which will seal at the more desirable lower temperatures, i.e., about 80.degree. to 90.degree.C., are usually noncrystalline and, hence, poor barriers.
In addition good barrier properties and heat sealability, coated packaging films must have good machinability properties for easy handling. Thus, it is well known in the art to incorporate surface effect additives into the film coating systems to improve slip characteristics, anti-blocking properties and electrostatic conductivity. These additives, however, may cause the film's minimum heat seal temperature to increase.
To obtain an optimum combination of properties in packaging materials, therefore, it has been found that film substrates carrying more than one copolymer coating are desirable. British Pat. No. 1,234,143 to Phillips et al., for example, teaches the preparation of a heat-sealable, moisture-proof sheet material comprising a flexible base sheet having on each surface thereof a sub-coating and a top coating, both of which include a vinylidene chloride copolymer. Each coating layer is applied as an aqueous dispersion followed by a conventional drying step. Alternatively, the top coating may be applied from a volatile organic solvent solution containing about 30% solids.
The use of an aqueous dispersion containing a vinylidene chloride copolymer as a sub-coating and a top coating imply a copolymer layer adjacent the substrate which, of necessity, ordinarily contains a wetting agent. In high humidity atmospheres water vapor, usually absorbed at the interface between the film substrate and the coating layer, may substantially reduce the adhesion of the coating composition to the substrate. Top coating compositions which contain wetting agents, on the other hand, may interfere with heat sealing the film by raising minimum heat seal temperatures. It is also known that wetting agents, being generally poor barrier materials, may offer moisture pathways through the coating composition. Even when the top coating composition is applied from a suitable organic solvent solution, the presence of wetting agents in the sub-coating may degrade the barrier properties as well as the resultant high humidity adhesion properties of the film.
It has been recognized, in applications where a packaging film having extremely high barrier properties is preferred, that it is not generally practicable to apply the sub-coating to the base sheet from an organic solvent solution. Difficulties may occur because the preferred copolymers, i.e., those having a high vinylidene chloride content, may present solvent solubility problems. Moreover, even where the preferred copolymer is soluble in the solvent and is applied to the base sheet to form a suitable sub-coating, the subsequent top coating may interfere with the evaporation of solvent from both coating layers during the final heating and drying step. There can result, then, a film product having a double copolymer coating with an unacceptable amount of solvent retained within each coating layer, i.e., greater than 500 ppm for each constituent based on the total weight of coated film.
The method of the present invention can be used to overcome the problem of solvent retention and to offer a substantially improved high barrier packaging film.